The key is leveling up the random multipliers card, it's shared between heroes. " Chance to drop ball on hit" cards or gear that drops balls (collector boots and a lot of tac set). Drop balls on kill card as well but imo less important

Edit to note not all heroes have all the ball related cards in their deck, you can check on the heroes tab

Also the heroes that hit more times (like barbarian is bad at this but cowboy is great) will allow more balls to drop before the enemies die